1. Field of the Invention
The present invention relates generally to a method for filling an electrical device using insulating filler and more particularly to a method for filling an electrical device for medium and high voltage applications, i.e. electrical devices having voltages greater than 1000V, using a multi-component insulating filler.
2. Description of the Related Art
Electrical devices for medium and high voltage applications, in particular for transmission and/or distribution lines, such as circuit breakers, power transformers and measuring transformers, insulated switch-gears, surge arresters, cable terminations, pole heads, bushings, and insulators include free volumes that are designed to be filled with insulating materials. These insulating filler materials, which can be solid, semi-solid, liquid or gaseous materials, primarily function to guarantee the dielectric strength among the parts of the device having different voltages in the presence of electrical stresses due to the normal functioning of the device itself.
Dielectric oils are one example of liquid insulating fillers that are not entirely satisfactory for this purpose for various reasons. For example, they require the use of compensating volumes to cope with any expansion of the insulating oil following changes in temperature.
In the case of gaseous insulating fillers, such as sulphur hexafluoride (SF6) or nitrogen and/or related mixtures, it is necessary to use gas pressure monitoring devices as well as gas filling devices in order to keep the dielectric oil's insulating capacities unchanged. Moreover, in both the case of gaseous and liquid fillers, it is necessary to adopt special measures and to equip the electrical devices with safety systems in order to avoid and/or indicate filler leakages and discharges. Leakages and consequent discharges of the dielectric filler can not only lead to malfunctioning of the electrical device but to environmental pollution problems as well. Gaseous insulating fillers clearly affect the electrical devices in terms of structural complexity and the overall reliability of the insulation system.
In some applications single component or multi-component solid or semi-solid materials are used as insulating fillers, for example polyurethane, silicone foams or rubbers, silicone gels or polyurethane gels etcetera. Although these solid or semi-solid fillers make it possible to overcome the aforementioned drawbacks due to their intrinsic properties, they require special attention during installation in order to achieve optimal insulating performance. A typical drawback of solid or semi-solid fillers resides in the fact that they require accurate controls to verify that there is good adhesion between the walls of the electrical device and the filler itself. Indeed, in case of defective and non-homogeneous adhesion to the walls of the device, destructive electric discharges may generate due to air filtering. These electric discharges could cause the electric device to breakdown.
One of the solutions adopted to overcome this problem is to treat the surfaces of the electrical device with primers in order to facilitate good and homogeneous adhesion between the walls and the filler. Such treatments are expensive and complicated especially in the case of electric devices having complex geometry and in particular when functional elements such as cables, mechanical rods, and connections subject to voltage are present inside them.
Another problem related in particular to the use of solid insulating fillers is the fact that they generally have high thermal expansion coefficients associated with a reduced ability to insulate. These properties make these materials extremely sensitive to thermal changes and, if special measures are not adopted, limit their use to thermally stable environments. For example, cross-linked silicone elastomers can be used as insulating fillers for electrical applications. These materials have a very high thermal expansion coefficient (about 10.sup.3 C.sup.-1) and a very low compressibility, comparable to the compressibility of liquids. In application, because the silicone elastomer is contained in enclosed spaces and subject to dilation caused by possible heating, damage to the electrical device can occur.
In the specific case of semi-solid insulating fillers (e.g., silicone gels or polyurethane gels) since these materials are generally made up of two or more components that have different chemical-physical characteristics from one another, the filling method demands particular care both during preparation of the filler and while the filler is being injected into the device. In particular, in order to obtain a final product that fully satisfies performance requirements, it is necessary to homogenize and mix the components forming the filler correctly. If this is not done, unsatisfactory polymerization of the material may result inside the device resulting in negative consequences for the dielectric strength of the system. Moreover, unlike other filler materials, such as foams or resins, gels are semi-solid materials that behave like a highly viscous material before polymerization and like a viscoelastic rubber material thereafter. This makes it necessary to use special injection devices.
Moreover, the fact that the filling method must ensure that the filler fills up all the spaces available and adheres to the walls of the device without performing any surface treatments should not be undervalued. Further measures need to be adopted in order to satisfy these conditions, since gels are mechanically weak materials that nevertheless have low fluidity.